1. Field of the Invention
The present invention relates to a two-dimensional digital data acquisition element and a holographic storage apparatus.
2. Related Art
A new optical disk system called holographic storage is proposed as a DVD (Digital Versatile Disc) in a generation after the next generation, and makers announcing officially its commercialization plans are also appearing (see, for example, H. J. Coufal, Holographic Data Storage (Sprihger, Berlin, 2000).
The holographic storage has a feature that a higher data density and fast readout are possible because a laser having a short wavelength of approximately 400 nm is used and two-dimensional digital data encoded in the holographic layer at high density as interference fringes are read fast.
The two-dimensional image data acquisition element is required to have an extremely high frame rate of, for example, at least 1000 fps to fast read out two-dimensional digital image data.
For acquiring the two-dimensional digital image data accurately, it is ideal that a pixel of the two-dimensional image data acquisition element such as a CMOS sensor corresponds to a unit data area forming two-dimensional digital image information in one-to-one correspondence.
However, the cost for adjustment is required for position adjustment in the optical system in micron order. In addition, every time an optical disk having encoded two-dimensional digital image information is placed on a drive, its position changes. Therefore, it can be said that strict position adjustment in the optical system is extremely difficult.
In addition, a lens system included in the optical system is required to be small in size and low in cost. Therefore, it cannot be avoided that optical aberration occurs in two-dimensional digital data. It is difficult to implement to associate a unit data area with a pixel strictly in one-to-one correspondence.
As its countermeasure, a technique called “oversampling” is used. In the “oversampling” technique, the pixel pitch of the two-dimensional image data acquisition elements is downsized to 1/N as compared with the pitch of the unit data areas which form the two-dimensional digital image and information of a unit data area is acquired by using N2 pixels. (See, for example, Mark Ayres, Alan Hoskins, and Kevin Curtis, “Image oversampling for page-oriented optical data storage,” Applied Optics, Vol. 45, Issue 11, pp. 2459-2464.)
Even when this oversampling technique is used, the pixel output assumes a middle value between “1” (high level) and “0” (low level) in pixels other than pixels for which the above-described accurate digital data are acquired. Although the data quantity of the unit data area in the two-dimensional image data acquisition element is one bit, an output of M (≧2) bits is required to acquire and output pixel output data of the middle level.
In addition, the light intensity in the unit data area varies under the influence of an output power variation of short-wavelength laser light applied to an optical disk as a reference beam in order to obtain a holographic image and an optical loss in an optical system including the optical disk. Even in outputs of pixels for which digital data can be acquired accurately, the value of “1” (high level) varies. To solve this variation issue, the output of M (≧2) bits is required although the data quantity of the unit data area in the two-dimensional image data acquisition element is only one bit.
For reconciling the M-bit output and the above-described fast readout, it is necessary to use a special CMOS image sensor as the two-dimensional image data acquisition element.
For example, it becomes necessary to use a multi-line column CDS (Correlated Double Sampling)/ADC mounting CMOS sensor increased in speed by using an intra-pixel ADC (Analog-Digital Converter) mounting CMOS sensor in which fast operation is made possible by conducting A/D conversion parallel in all pixels or activating column CDS/ADC's which operate by taking a row as the unit, in parallel in a plurality of rows.
In addition, it is necessary to make outputs of an M-bit ADC faster by using a multi-line digital data parallel output structure which outputs, in parallel, outputs of the M-bit ADC corresponding to K-pixels. The number of output pins becomes M×K. For example, in a 10-pixel parallel readout device having an ADC resolution of 10-bits, M×K=10×10=100 pins are needed.
Therefore, the CMOS sensor chip is difficult to reduce the size and price because of its specialty. Furthermore, the CMOS sensor package needs to have a large number of pins because of its multi-line output structure, and reduction in size and price is difficult in the same way. A multi-line input I/O is needed by an external circuit which receives the output of the CMOS sensor, conducts image signal processing, and reconstructs two-dimensional digital data obtained. In addition, it is necessary for the external circuit to process two-dimensional M-bit information fast. Specifications required of the external circuit are strict, and it is difficult to reduce the size and price of the external circuit in the same way.